Mobile work zones are used for many road maintenance operations such as roadway striping, sweeping, and minor pot-hole repair. These types of work zones are slow moving with respect to normal traffic and can surprise an inattentive traveller. With the increasing use of cell phones and other devices that are distracting drivers, there is a growing need for an additional method to alert travellers approaching slow moving mobile work zone operations. One possible counter-measure is a mobile work zone alarm system. This report describes the first field test of mobile work zone alarms in the United States. This project analyses two types of devices: an Alarm Device and a Directional Audio System (DAS). Examples of a DAS include parametric speaker arrays and the Long Range Acoustic Device (LRAD) (LRAD, 2014). The LRAD was the DAS used for testing in this research project. Each device is attached to a Truck-Mounted Attenuator (TMA). Five different setups were tested in the field based on various operating modes: Control setup with no alarm system, Alarm Manual, Alarm Actuated, DAS Continuous, and DAS Actuated. In the manual operating mode, the TMA driver manually activates the alarm while the actuated mode uses an actuation system to trigger the alarm based on the speed and merging distance of an approaching vehicle. The evaluation included sound level testing, spectral analysis to investigate the distinctiveness of the alarm sounds, analysis of merging distances and speeds, anecdotal observations of driving behaviour, and investigation of the alarm actuations. These investigations provided insight into the effectiveness of the alarm systems and led to recommendations for improvements to the systems. Sound level tests were performed for both systems in a parking lot at various distances from both inside and outside of a vehicle. Additional sound level tests were performed in the field in instrumented vehicles. Sound levels were also measured from inside the TMA vehicle to analyse worker sound exposure. The results from the tests indicated that the sound levels were in accordance with standards established by the Occupational Safety and Health Administration (OSHA) and National Institute for Occupational Safety and Health (NIOSH) except for the extreme case of standing three feet directly behind the unit for an extended period of time. This situation is all but impossible to occur in a moving work zone. In addition to the sound level tests, the alarm sounds were evaluated for distinctiveness through the use of spectral analysis. The results of the spectral analysis indicated that the DAS produced a more distinctive sound that was better able to overcome background road noise than the Alarm Device. The luminance levels of the lights on both alarm systems were also measured and found to be comparable thus ensuring that they did not affect driver behaviour and bias the results. Driving behaviour was the main measure of warning system effectiveness in increasing mobile work zone safety. Factors such as average merging distance, standard deviation of merging distance, average speed, and other observed driving behaviours were analysed. The Safe Stopping Sight Distance (SSD) from the AASHTO Green Book (AASHTO, 2011) was used as the definition for desirable driving behaviour. SSD represents the smallest distance a vehicle could stop safely assuming a conservative deceleration rate and perception/reaction time. SSD was calculated as 600 ft from the TMA vehicle for a 60 mph speed differential. Therefore, desirable driving behaviour was defined as merges that occurred at distances greater than 600 ft from the TMA vehicle. Using the 600 ft threshold, each vehicle’s merge distance was measured, and vehicle merges that occurred within 600 ft were further analysed for average vehicle speeds and driving behaviour observations. The first five rows of Table ES.1 show warning setup performance measures. In comparing average merging distances by setup, all setups were observed to result in an increase in merging distance except for the Alarm Actuated setup. The standard deviation of merge distance and average speed were observed to decrease only in the DAS Continuous setup, indicating that DAS Continuous setup may be the most effective setup for improving mobile work zone safety. However, some undesirable driver behaviours were observed with the DAS setups. Instances in which some drivers had sudden reactions, such as braking or swerving, were observed with the DAS Actuated setup. It is unclear whether these behaviours were due to the actuation of the mobile work zone alarm. While the DAS Continuous setup was in operation, some drivers were observed passing the TMA on the shoulder, giving the TMA an additional amount of space while passing. MoDOT personnel have indicated that drivers sometimes pass the TMA on the shoulder during routine operations, so this behaviour may not be due to the presence of the mobile work zone alarm. False alarm and false negative statistics are an important part of investigating activation systems used in alarm setups. By using the audio data with merging speeds and distances, each vehicle merge was evaluated as either a successful alarm activation, successful negative, false positive, or false negative. Some general causes of false alarms and false negatives were horizontal and vertical curves in the roadway as well as movement of the TMA vehicle. The research demonstrated that mobile work zone alarms have the potential to be an effective tool in improving safety by warning drivers. In determining which system to use, agencies should consider a variety of factors such as performance, cost, and maintenance requirements as shown in Table ES.1. Since this project was an initial test to investigate the feasibility of mobile work zone alarms, further refinements to the systems, such as modifications to the alarm sound or warning message, could improve system effectiveness. (Author/publisher)
Samenvatting